2. Endocrines and Populations 263 



The general plan of the following discussion will be to explore first the 

 ability of purely social factors to affect adrenocortical and reproductive 

 function. Next the relation between population density and adrenocortical, 

 reproductive, and other functions as well as alterations in resistance to 

 disease, will be investigated. In general the plan will proceed from popula- 

 tions of fixed size in the laboratory to freely growing populations in the 

 laboratory, and finally to natural populations. Under each of these experi- 

 mental categories adrenocortical function, reproductive function, disease 

 resistance, and mortality will be discussed with all appropriate experiments 

 and species. The effect of population density on growth will be discussed 

 where appropriate, and the effects of food and other environmental factors 

 on social interactions and endocrine function will be discussed. It is not 

 possible to stay strictly within this framework, as it is somewhat artificial, 

 but it does seem to offer the most logical means of presenting the available 

 information as it progresses from the most artificial but most highly con- 

 trolled experiments to natural populations which are controlled with great 

 difficulty, if at all. 



II. Endocrine Responses to Social Pressures and to Population Density 



A. Experiments in the Laboratory with Populations of Fixed Size 



One of the basic tenets in the theory that physiological feedback mecha- 

 nisms can regulate population growth is that a fundamental regulating 

 factor must be present and active in all populations. Whether or not this 

 particular factor is the proximate factor limiting population growth in a 

 given instance is not important if it is universally present. The only known 

 element common to all populations is social interaction, or intraspecific 

 competition. Basically, competition depends on the behavioral characteris- 

 tic of the species, but some sort of social organization or mutual intolerance 

 is exhibited by all species of mammals. 



1. Social Factors, Adrenocortical and Reproductive Functions 



Social interactions may arise from two kinds of situations: one in which 

 there is invasion of the private territory of one animal by another; and 

 another in which there is conflict involved in the establishment and mainte- 

 nance of a hierarchical situation. The first requirement of the hypothesis 

 that physiologic mechanisms can and do control population growth is to 

 show that purely behavioral or social interactions, acting through the 

 central nervous system, can induce endocrine responses, especially of the 

 pituitary-adrenocortical and reproductive systems, and to be able to rule 



